The present invention relates to an air-fuel ratio control system for an internal combustion engine, which detects the oxygen density in the exhaust gas of, for example, an automobile, to control the air-fuel ratio thereof based on the thus obtained oxygen density.
In recent years, feedback control of the air-fuel ratio immediately after starting an engine has been demanded for the purpose of improving the fuel cost of an internal combustion engine (engine) and for the purpose of promoting cleaning of the exhaust gas. Usually, in such feedback control of the air-fuel ratio, the oxygen density in an exhaust gas is detected by using an oxygen sensor and the mixture ratio of fuel to be supplied to the engine is adjusted on the basis of the detected oxygen density.
Usually, an oxygen sensor used in the above feedback control has an output characteristic which largely depends on temperature and a suitable output may be obtained in a specific temperature range where the sensor element is activated. A heater is thus attached to the body of the sensor to quickly activate the sensor element after starting the engine.
Further, a device (see Japanese Patent Laid-Open Publication No. 61-274249) for use in a system having such a sensor has been proposed, in which power to be initially supplied to the heater of the oxygen sensor when starting the engine is determined on the basis of the temperature of the oxygen sensor at the time of starting.
Since, however, these are all constructed so that an electric conduction to the heater begins at the time of or after starting the engine, they are not satisfactory in achieving a sufficient element temperature for starting the feedback control of the air-fuel ratio at the time when the engine is started. Thus there has been a problem in that a favorable air-fuel ratio control cannot be provided at the time when the engine is started.
In addition, a device (SAE No. 900503) has been proposed, in which preheating is performed before starting the engine. In this art, a seat sensor or the like is attached to the seat of an automobile and the temperature of a catalyst with a heater is elevated to its activation temperature when the seat sensor is turned on upon taking of the seat by a driver. By this method, however, the amount of power to be supplied up to the time of starting becomes large, and, if the battery is of a low voltage or of a small capacity, it is not desirable in that the battery may be exhausted.